I have no concrete answers, so obviously i can only rely on my own experience...
With that said, to answer the steam question... ATI hasn't had a competitive graphics card (speed-wise) arguably since the x800 series. The 2xxx and 3xxx were not very fast, although they made up for it somewhat in sweet features (HDMI passthrough, bringing back the AIW brand, hardware accelleration for mpeg 4 and h.264, etc). Only until a few months ago when they came out with the 48xx series did they start kicking nvidia's ass in speed again.
Of particular note from the survey, i don't even see the Radeon 4850 or 4870 series at all in that list. And to be fair, i don't see the geforce 260 or 280 either. The survey started November 2007. it was updated last month but who knows if they were really taking new info for a whole year or no. The fact that no new graphics card is showing up must mean that the sample was maybe only a month or two?
My own personal stance has always been to find the best product at the time. I could care less about popularity. The way I see it, popularity has never been an indication of quality. Previous to my Radeon 4850 was the 8800gtx. It was very fast but horribly horribly buggy. I'm so happy i'm back to ATI. The last vestige of crappy nvidia hardware (and the cause of frying all my memory and a weekly lock-up) is my 680i motherboard which i'm looking forward to replacing some day.